What I want from you guys, is to post up any convincing reasons on why the kilt should be allowed to a school dance that I can add to my letter.
Hmmm... not knowing what you've already got. One thing I would certainly suggest is to point out the heritage aspect of wearing a kilt. That starts to put it on a PC footing, and most school admins will embrace anything to be PC...

Also have LOTS of pictures (this was advised before). I'm thinking especially a couple of the pics from the "2 kilts, 10 looks" thread. If you have pics of yourself in kit, that would be helpful as well.

Good luck!